Aug 9, 2015
The Moon: As a matter of fact, it's all dark

Sen—What an astonishing picture, or sequence of pictures, we saw this week of the Moon passing in front of the Earth. Taken by NASA’s Deep Space Climate Observatory satellite (DSCOVR) from a distance of 1.6 million km (a million miles) it showed the lunar far side, most of which we never get to see directly from home because the Moon always keeps the same face turned towards us.

That is because the Moon is tidally locked, making one rotation on its axis for every orbit that it makes of the Earth. It is sometimes erroneously referred to as the dark side of the Moon, though as the voice famously says on Pink Floyd’s legendary recording, ‘There’s no dark side of the Moon really; as a matter of fact, it’s all dark.’

To put it more scientifically, all parts of the Moon experience around 14 days of daylight, then 14 days of darkness, as it revolves around the Earth and different areas catch the sunlight. You can see this for yourself as you watch the phase of the Moon change. When it is a crescent, most of the illuminated hemisphere will be on the far side; when it is Full Moon, the far side is indeed in darkness. (Some crater interiors at the South Pole never see sunlight at all, because it is blocked by their walls, and within them will be found quantities of water ice.)

When I first saw that DSCOVR sequence of the Moon passing in front of the Earth, the far side certainly did look dark. But then I realised that we were viewing the fully-illuminated side of the Moon because it was receiving the same sunlight, from the same direction, as the Earth. But whereas our waterworld, and its clouds, appeared bright and colourful in the sunshine, the Moon, by contrast, was a dull grey.

I say fully-illuminated, but if you look carefully, you’ll see that the satellite was not quite in line with the Sun, and so is giving us just a peep at part of the night side of both worlds, along the right limbs. Note how black that narrow band appears on the Moon against the backdrop of the Earth in our main picture, above.

NASA's Galileo probe looked back to take this snapshot of the Earth and Moon together on Dec. 16, 1992. Image credit: JPL/NASA

The reason the illuminated Moon is much darker than the Earth is the fact that the surface is made up of dark igneous rock. The mountains are mainly anorthosite, while the darker maria, or ‘seas’, which give the Man in the Moon its distinctive face, are basalt that flooded the surface to produce vast lava plains, billions of years ago. The reflectivity of a body is known as its albedo, and the albedo of the Moon is low.

It is a sobering thought that we had no real idea of what the far side of the Moon was like until October 1959, when the Soviet probe Luna 3 sent back the first pictures. I recall being aware of this important advance as I approached my seventh birthday. The pictures, which were crude by today’s standards, were released to a live broadcast of Patrick Moore’s BBC TV show The Sky at Night, which was then only six months into its run (it is still going!).

Crude they may have been, but the images immediately revealed that the Moon’s far side was quite different from the side facing Earth, There were notably fewer maria, or large basins, and much more extensive cratering. Today we know that just one per cent of the far side is covered by maria, compared to nearly a third of the near side.

GRAIL’s study, made in a little over a year, told us was that the lunar crust on the near side is thinner than on the far side. That facing us is about 30 km thick on average, whereas on the far side the crust is up to 60 km deep. Perhaps the thinner crust on the near side allowed more volcanic eruptions long ago, so that lava found it easier to reach the surface.

I started by saying that most of the far side is hidden from direct view of astronomers on Earth. In fact they have been able to see some of the far side since the invention of the telescope because the Moon undergoes a slight rocking motion as it orbits that allows us to peer over different regions of the lunar limb at different times.

The Moon is caught rising above the Pacific in this image from the Rosetta spaceprobe in March, 2005. Image credit: ESA/Emily Lakdawalla

This rocking, an effect known as lunar libration, means that around 59 per cent of the Moon’s surface is visible from Earth at some stage or other, though features brought into view over the limb will always appear at a very oblique angle. The phenomenon also clearly affects the appearance of some familiar parts of the moon, such as Mare Crisium, the Sea of Crises, a dark plain that looks more compressed and closer to the limb at certain times than at others.

DISCOVR’s remarkable images of the Moon and Earth together are the latest in a number that have been taken by remote space probes. NASA’s Voyager 1 took the first picture of our companion worlds, as a pair of crescents, in September 1977 as it headed out to make a grand tour of the Solar System.

The Galileo probe took a similar look back in December 1992, on its way to explore Jupiter, and Rosetta, which is now orbiting Comet 67P/Churyumov-Gerasimenko, caught the Moon rising over the Earth in a haunting shot in March 2005. Several other planetary missions have captured their own views—Messenger at Mercury even caught a lunar eclipse—and they have all helped give us a new perspective on our place in space.